cmd/go: permit C files if the package uses SWIG

They will be compiled and added to the archive, just as though the
package used cgo.  In effect all SWIG packages now use cgo anyhow.

Change-Id: I5d5a28ed0ec4295f24036b2834218bc980f080d0
Reviewed-on: https://go-review.googlesource.com/10146Reviewed-by: Minux Ma <minux@golang.org>

src/cmd/go/pkg.go

@@ -673,10 +673,10 @@ func (p *Package) load(stk *importStack, bp *build.Package, err error) *Package
 	p.Target = p.target
 
 	// The gc toolchain only permits C source files with cgo.
-	if len(p.CFiles) > 0 && !p.usesCgo() && buildContext.Compiler == "gc" {
+	if len(p.CFiles) > 0 && !p.usesCgo() && !p.usesSwig() && buildContext.Compiler == "gc" {
 		p.Error = &PackageError{
 			ImportStack: stk.copy(),
-			Err:         fmt.Sprintf("C source files not allowed when not using cgo: %s", strings.Join(p.CFiles, " ")),
+			Err:         fmt.Sprintf("C source files not allowed when not using cgo or SWIG: %s", strings.Join(p.CFiles, " ")),
 		}
 		return p
 	}